% DESCRIPTION:
%     Subscript for the first-order k-Wave simulation functions to create
%     the absorption variables.
%
% ABOUT:
%     author      - Bradley Treeby
%     date        - 26th November 2010
%     last update - 13th October 2017
%       
% This function is part of the k-Wave Toolbox (http://www.k-wave.org)
% Copyright (C) 2010-2017 Bradley Treeby

% This file is part of k-Wave. k-Wave is free software: you can
% redistribute it and/or modify it under the terms of the GNU Lesser
% General Public License as published by the Free Software Foundation,
% either version 3 of the License, or (at your option) any later version.
% 
% k-Wave is distributed in the hope that it will be useful, but WITHOUT ANY
% WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
% FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License for
% more details. 
% 
% You should have received a copy of the GNU Lesser General Public License
% along with k-Wave. If not, see <http://www.gnu.org/licenses/>.

% define the lossy derivative operators and proportionality coefficients
if strcmp(equation_of_state, 'absorbing')
    
    % convert the absorption coefficient to nepers.(rad/s)^-y.m^-1
    medium.alpha_coeff = db2neper(medium.alpha_coeff, medium.alpha_power);

    % compute the absorbing fractional Laplacian operator and coefficient
    if ~(isfield(medium, 'alpha_mode') && strcmp(medium.alpha_mode, 'no_absorption'))
        absorb_nabla1 = (kgrid.k).^(medium.alpha_power - 2); 
        absorb_nabla1(isinf(absorb_nabla1)) = 0;
        absorb_nabla1 = ifftshift(absorb_nabla1);
        absorb_tau = -2 .* medium.alpha_coeff .* medium.sound_speed.^(medium.alpha_power - 1);
    else
        absorb_nabla1 = 0;
        absorb_tau = 0;
    end
       
    % compute the dispersive fractional Laplacian operator and coefficient
    if ~(isfield(medium, 'alpha_mode') && strcmp(medium.alpha_mode, 'no_dispersion'))
        absorb_nabla2 = (kgrid.k).^(medium.alpha_power-1); 
        absorb_nabla2(isinf(absorb_nabla2)) = 0;
        absorb_nabla2 = ifftshift(absorb_nabla2);            
        absorb_eta = 2 .* medium.alpha_coeff .* medium.sound_speed.^(medium.alpha_power) .* tan(pi .* medium.alpha_power / 2);
    else
        absorb_nabla2 = 0;
        absorb_eta = 0;
    end
        
    % pre-filter the absorption parameters if alpha_filter is defined (this
    % is used for time-reversal photoacoustic image reconstruction
    % with absorption compensation)
    if isfield(medium, 'alpha_filter')
                
        % update command line status
        disp('  filtering absorption variables...');        
        
        % frequency shift the absorption parameters
        absorb_nabla1 = fftshift(absorb_nabla1);
        absorb_nabla2 = fftshift(absorb_nabla2);
                        
        % apply the filter
        absorb_nabla1 = absorb_nabla1 .* medium.alpha_filter;
        absorb_nabla2 = absorb_nabla2 .* medium.alpha_filter;

        % shift the parameters back
        absorb_nabla1 = ifftshift(absorb_nabla1);
        absorb_nabla2 = ifftshift(absorb_nabla2); 
           
    end    

    % modify the sign of the absorption operators if alpha_sign is defined
    % (this is used for time-reversal photoacoustic image reconstruction
    % with absorption compensation)
    if isfield(medium, 'alpha_sign')
        absorb_tau = sign(medium.alpha_sign(1)) .* absorb_tau;
        absorb_eta = sign(medium.alpha_sign(2)) .* absorb_eta;
    end
    
elseif strcmp(equation_of_state, 'stokes')
    
    % convert the absorption coefficient to nepers.(rad/s)^-2.m^-1
    medium.alpha_coeff = db2neper(medium.alpha_coeff, 2);

    % compute the absorbing coefficient
    absorb_tau = -2 .* medium.alpha_coeff .* medium.sound_speed.^(medium.alpha_power - 1);

    % modify the sign of the absorption operator if alpha_sign is defined
    % (this is used for time-reversal photoacoustic image reconstruction
    % with absorption compensation)
    if isfield(medium, 'alpha_sign')
        absorb_tau = sign(medium.alpha_sign(1)) .* absorb_tau;
    end    
    
end